The present invention relates to a color cathode ray tube, and more particularly to a so-called flat panel type color cathode ray tube which is provided with a panel whose inner surface is coated with a phosphor layer and whose outer surface has the radius of curvature extremely larger than that of an inner surface thereof.
Recently, color cathode ray tubes called the flat face type or the flat panel type have been widely adopted as picture tubes for television receivers or monitor display tubes of personal computers or the like.
Generally, a glass-made envelope of a color cathode ray tube is composed of a panel which forms a display part (screen), a narrow-diameter neck and a funnel in a funnel shape which connects the panel with the neck. A phosphor screen (phosphor film) coated with a phosphor in three colors is formed on the inner surface of the panel and a shadow mask which works as a color selection electrode is disposed close to this phosphor screen.
Further, an electron gun which emits three electron beams arranged in in-line is accommodated in the inside of the neck and three electron beams emitted from the electron gun are made to pass through beam apertures formed on the shadow mask and are impinged on respective phosphors to reproduce color images.
Recently, this type of color cathode ray tube has an outer surface of the panel thereof flattened so as to enhance the visibility thereof. Particularly, this flattening of the panel is widely adopted with respect to color cathode ray tubes having large screens. The color cathode ray tubes having such flattened panels are called the flat panel type color cathode ray tubes or the flat-surface panel type color cathode ray tubes.
As literatures which disclose conventional techniques on this kind of flat panel type color cathode ray tube, Japanese Laid-open Patent Publication 45667/1999 and Japanese Laid-open Patent Publication 238475/1999 can be named.
FIG. 6 is a schematic cross-sectional view showing a structural example of an essential-part of a flat panel type color cathode ray tube. In the drawing, a panel 1 is joined to a periphery of a large diameter which constitutes one end of a funnel 3 and the other end of the funnel 3 which gradually narrows its diameter in a funnel shape is connected to a neck not shown in the drawing.
An outer surface PO of the panel 1 which forms a phosphor screen (screen) having an approximately rectangular shape and coated with a phosphor layer 4 on an inner surface thereof has a curved surface which is substantially a flat surface, wherein the radius of curvature of a curved surface of the inner surface PI is set to be smaller than the radius of curvature of the outer surface PO for maintaining the mechanical strength of a glass-made envelope.
A shadow mask 6 which constitutes a color selection electrode is disposed in the vicinity of this phosphor layer 4. A large number of electron beam passing apertures 6a are formed in the shadow mask 6. The shadow mask 6 is welded to a mask frame 7 and is held to the inner surface of a side wall of the panel 1 by way of a suspension mechanism not shown in the drawing.
In view of the manufacturing cost and the easiness of manufacturing, with respect to the above-mentioned flat panel type color cathode ray tube, the outer surface (also called xe2x80x9cfacexe2x80x9d) of the panel is set to have a large radius of curvature, that is, the outer surface is set to an approximately flat surface, while the inner surface on which a phosphor layer is formed is set to have a relatively small radius of curvature to a degree that the feeling of a flatness of a displayed image is not spoiled when the display screen is seen from the outer surface.
In manufacturing the flat panel type color cathode ray tube, it is easy to approximate the shape of the outer surface of the panel to the flat surface. However, to approximate the inner surface of the panel to the flat surface, the thickness of the whole panel must be considerably increased to increase the mechanical strength of the glass-made envelope. Therefore, this is not practical in view of the increase of the weight of the cathode ray tube, the increase of cost and the like.
Further, on the other hand, with respect to the shadow mask which is not a color selection electrode of a so-called tension type, it is necessary to form a mask surface thereof with a certain degree of curvature while eliminating a completely flat surface to make the shadow mask stand by itself. Since the manufacturing of a shadow mask having large radius of curvature by a press molding is technically limited, it is necessary to give a given curvature to the shadow mask and simultaneously to give a given curvature to the inner surface of the panel.
As shown in FIG. 6, the curvature of the inner surface Pl of the panel 1 is larger than the curvature of the outer surface PO (the radius of curvature of PI being smaller than the radius of curvature of PO) and the shadow mask 6 approximately follows the shape of the curved surface (warp) of the inner surface Pl of this panel so that the flatness is deteriorated at the peripheries in the long axis side (X axis) on the phosphor layer 4.
In the flat panel type which largely warps the inner surface, there has been a problem that the larger the panel size of the color cathode ray tube (equal to or more than xe2x80x98nominal 17 inchesxe2x80x99 (effective screen diagonal diameter being 41 cm)) becomes, the feeling of flatness in the short sides of the screen (end portions in the X axis direction) is worsened due to the aspect ratio of the screen.
Particularly, when the radius of curvature of the outer surface of the panel along the diagonal axis of the screen is not less than 10000 mm and the radius of curvature of the inner surface of the panel along the diagonal axis of the screen is not more than 7000 mm, the peripheries of the effective surface of the phosphor layer which is coated on the inner surface of the panel is made geometrically straight. In such a case, when it is viewed from a viewing position (limited position) which is slightly away from the panel of the color cathode ray tube, the peripheries of the effective surface appears in a barrel shape and hence, the feeling of flatness of the screen is damaged. To cope with such a problem, it is effective to form the shape of the effective region of the phosphor layer in a geometrically pincushion (bobbin) shape. Further, due to such a measure, even when the effective diameters of the screen in the X-axis direction and in the Y-axis direction are fixed, it becomes possible to increase the effective diameter in the diagonal direction. However, the realization of such a measure has problems which will be explained hereinafter.
FIG. 7 is an explanatory view of the shape of the phosphor screen showing the coated shape of the approximately rectangular phosphor layer when the side portions are formed in a linear shape or a pincushion shape. In the drawing, numeral 4 indicates the phosphor layer, numeral 4a indicates a contour of the effective region of the linear-shaped phosphor layer, and numerals 4b and 4bxe2x80x2 indicate the contours of the effective regions of the pincushion-shaped phosphor layer.
FIG. 8 is a cross-sectional view showing a corner portion B of the effective region of the linear-shaped or pincushion-shaped phosphor layer shown in FIG. 7 in an enlarged form. In the effective region of the phosphor layer shown in FIG. 7, an axis which is extended in the main scanning direction (in-line arrangement direction of electron beams) and passes the tube axis is set as an X axis, an axis which is extended in the direction perpendicular to the main scanning direction and passes the tube axis is set as a Y axis and an axis which is extended in the diagonal direction and passes the tube axis is set as a D axis.
The contour 4a of the effective region of the linear-shaped phosphor layer is approximately accommodated in a front region of the panel 1. However, with respect to the contour 4b of the effective region of the pincushion-shaped phosphor layer, when the effective region is ensured along the X axis and the Y axis as much as possible, the effective diameter along the D axis is increased so that the phosphor layer at end portions in the diagonal axis direction of the effective region is extended to the inner surface region close to a skirt portion 1a of the panel.
This region close to the panel skirt portion 1a is called a blend curved-surface portion defined between the face plate portion and the skirt portion. The blend curved-surface portion has an extremely large curvature compared to the face plate portion and has an extremely large panel wall thickness compared to the face plate portion. In accordance with such a sharp change of the wall thickness of the panel, in a phosphor film coating process, the panel temperature, the distribution of film thickness, the back exposure condition and the like at the blend curved-surface portion are sharply different compared to those at the face plate portion. Further, in the coating process, since the incident angle of light in an exposure step of a phosphor pixel pattern is large particularly in the diagonal direction of the phosphor screen, the exposure illuminance is low and hence, the adhesive strength of the phosphor pixels (for example, dots) becomes weak and this gives rise to pixel defects (for example, dot omission phenomenon). Accordingly, the coating performance (yield) of the phosphor is lowered and the fabrication of the cathode ray tubes becomes difficult.
On the other hand, as in the case of the pincushion-shaped phosphor layer 4bxe2x80x2, when the effective diameter along the D axis is restricted so as to prevent the phosphor layer at the diagonal end portion from approaching the skirt portion 1a of the panel in the same manner as the linear-shaped phosphor layer 4a, the effective diameters in the horizontal direction and in the vertical direction respectively gradually become smaller toward the X axis and the Y axis. Accordingly, the area of the effective region of the pincushion-shaped phosphor surface 4bxe2x80x2 is shrunk compared with the linear-shaped phosphor layer 4a so that the image display ability (number of pixels) is reduced.
These have constituted tasks to be solved in the technical field of the color cathode ray tubes.
It is a typical object of the present invention to provide a color cathode ray tube which can increase effective diameters of a screen and can enhance the visual characteristics thereof by suppressing a phenomenon that the screen appears in a barrel shape when observed from a viewing position slightly away from a panel of the cathode ray tube.
It is another typical object of the present invention to provide a color cathode ray tube which can facilitate the fabrication of the cathode ray tube by enhancing the coating performance of a phosphor and can enhance the visual characteristics thereof by suppressing a phenomenon that the screen appears in a barrel shape when observed from a viewing position slightly away from a panel of the cathode ray tube.
According to a typical aspect of the present invention, an approximately rectangular screen effective region which is formed on a panel of a color cathode ray tube is formed in a pincushion shape. Further, curved peripheries which are protruded outwardly from the effective region are provided to end portions in the diagonal axis direction of the pincushion-shaped screen. Further specific constitutions of the present invention are as follows.
A panel which forms a screen effective region in an inner surface thereof and includes a phosphor layer whose contour as seen from the tube axis direction is approximately rectangular has an outer surface thereof formed into an approximately flat surface and the inner surface thereof curved with the radius of curvature smaller than the radius of curvature of the outer surface, and
when an axis which is extended in the main scanning direction of the screen effective region and passes the tube axis is set as an X axis, an axis which is extended in the direction perpendicular to the main scanning direction and passes the tube axis is set as a Y axis, and an axis which is extended in the diagonal direction and passes the tube axis is set as a D axis,
a coated shape of the phosphor layer is formed in an approximately pin (bobbin) shape, and at end portions in the diagonal axis direction of the phosphor screen, peripheries of long sides of the phosphor layer which sandwich the X axis and peripheries of short sides of the phosphor layer which sandwich the Y axis are connected so as to form corner portion peripheries curved with a given radii of curvature which are protruded outwardly from the effective region.
The equivalent radius of curvature on the outer surface of the panel along the D axis of the screen is set to not less than approximately 10000 mm and the equivalent radius of curvature on the inner surface of the panel along the D axis of the screen is set to not more than approximately 7000 mm.
Gaps on the X axis and the Y axis defined between lines which are connected such that the lines circumscribe respective corner portion peripheries of the approximately pin (bobbin)-shaped phosphor layer and peripheries of side portions of the approximately pin (bobbin)-shaped phosphor layer are set to 0.2 mm-3 mm.
The radius of curvature of curved peripheries of the corner portion peripheries of the phosphor layer is set to 1.5 mm-10 mm.
Due to such a constitution, a flat panel type color cathode ray tube having the favorable visibility and the enlarged effective region can be obtained.
Although operations and effects of the above-mentioned typical constitutions of the present invention will be explained in detail in the paragraphs of embodiment, it is needless to say that the present invention is not limited to those described in the embodiment and various modifications can be considered without departing from the technical ideas of the present invention.